linux-kernel - Re: [PATCH 4/5] context_tracking: Only define schedule_user() on !HAVE_CONTEXT_TRACKING

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Date:   Wed, 11 Nov 2020 23:22:38 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Phil Auld <pauld@...hat.com>,
        Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 4/5] context_tracking: Only define schedule_user() on
 !HAVE_CONTEXT_TRACKING_OFFSTACK archs

On Wed, Nov 11, 2020 at 03:34:58PM +0100, Peter Zijlstra wrote:
> On Tue, Oct 27, 2020 at 04:08:26PM +0100, Frederic Weisbecker wrote:
> > schedule_user() was traditionally used by the entry code's tail to
> > preempt userspace after the call to user_enter(). Indeed the call to
> > user_enter() used to be performed upon syscall exit slow path which was
> > right before the last opportunity to schedule() while resuming to
> > userspace. The context tracking state had to be saved on the task stack
> > and set back to CONTEXT_KERNEL temporarily in order to safely switch to
> > another task.
> > 
> > Only a few archs use it now and those implementing
> > HAVE_CONTEXT_TRACKING_OFFSTACK definetly can't rely on it.
> 
> powerpc64 and sparc64 only, right?

Exactly! I can precise that in the changelog.

Thanks.

> 
> > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > Cc: Marcelo Tosatti <mtosatti@...hat.com>
> > Cc: Paul E. McKenney <paulmck@...nel.org>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Phil Auld <pauld@...hat.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> > ---
> >  kernel/sched/core.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index c23d7cb5aee3..44426e5acde2 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4631,7 +4631,7 @@ void __sched schedule_idle(void)
> >  	} while (need_resched());
> >  }
> >  
> > -#ifdef CONFIG_CONTEXT_TRACKING
> > +#if defined(CONFIG_CONTEXT_TRACKING) && !defined(CONFIG_HAVE_CONTEXT_TRACKING_OFFSTACK)
> >  asmlinkage __visible void __sched schedule_user(void)
> >  {
> >  	/*
> > -- 
> > 2.25.1
> >